PeGASus Newsletter Issue #45 – Feb. 1994

  • Editorial Comments
  • In the Sky
  • he Astronomical Observations of James Cook
  • Monthly Meetings
  • Announcements
  • Picture Perfect
  • Astrology Survey
  • The Image Gallery

Prince George Astronomical Society Executive 1993/94

  • President Orla Aaquist 562-2131/964-9625
  • Vice President Bob Nelson 562-2131/563-6928
  • Secretary Jon Bowen 563-9869
  • Treasurer David Sundberg 562-5774/6655
  • Members at Large Ted Biech 562-2131/564-2838
  • Matthew Burke 964-3889

Nominated Positions

  • Technical Director Bob Nelson
  • Observing Director Jon Bowen
  • Promotional Director Orla Aaquist
  • PeGASus Editor Shannon Austman

The Observatory phone number is 964-3600. This is a party line, so if it rings busy, it does not imply that someone is at the observatory.

Editorial Comments

February is a short month and our second meeting of the year seems to fall on the heals of the first. Yet some exciting things have happened since the last newsletter.

The Hubble as been fixed. The image gallery this month illustrates the extent of the improvement. The three images were obtained directly from the JPL by Jon Bowen over the electronic network. We have commissioned a CNC student to retrieve all of the available images (or as many as our computer will hold) and they should be available by the next meeting.

Our long awaited digital setting circles arrived. They were tested by Bob, Jon, Matthew, Gill and Orla on a very cold February 2nd. Bob Nelson should be able to tell us more at the next meeting.

We have two rude cats. They wake me up every morning at 5:30 and demand food. The fat one sits on my shoulder and purrs. That wouldn’t be so bad if she didn’t also drool in my ear. The white amazon huntress almost rips my face off when she periodically (with a frequency of 1 per minute) dashes across the head of the bed, emitting a loud yowl in the process. I have tried to ignore them, but this is not possible because of the poor attitude of one white she-devil. So I am up at 5:30 every morning. This is a good time to give me a call if you want to chat.

In the Sky by Alan Whitman

Late in February Venus becomes visible very low in the south-west after sunset. It remains an evening star through to early October. On Sunday, March 13th, the thin two-day-old crescent moon will be about 10 degrees above the Venus. Jupiter continues to dominate the morning sky. By mid-March it rises before midnight.

Have you ever seen the zodiacal light? February and March are the best months to see it in the evening sky because of celestial geometry. You will need a fairly flat western horizon without any light pollution. The zodiacal light is not difficult to see. It’s brightest parts are actually brighter than the Milky Way, but it is a large softly glowing pyramid without definite boundaries. It is rather formless compared to the Milky Way, and so can be overlooked. Look for a triangular glow lying through Pisces and Aries (both zodiac constellations, of course) after the end of astronomical twilight. The broad base of the triangle is just above the western horizon while the vertex is farther south. Venus will indicate where the base of the zodiacal light will later appear (and fortunately, Venus will set before the end of twilight, so its brightness will not interfere). Look during the period from February 28th through March 12th when the early evening sky is moonless. The zodiacal light and its fainter extensions, the zodiacal band and the Gegen- schein, are unique in that they are only visible with the naked eye. They cannot be viewed in binoculars and telescopes because the field of view is too small to include the surrounding darker sky for contrast..

James Cook (The Astronomical Observations of James Cook) by Bob Nelson

In the summer of 1768 (Aug 26), Lieutenant (later Captain) James Cook, R.N., set sail from Plymouth, England in the barque Endeavour to make the first of his three great voyages of discovery into the Pacific. His ship was fully equipped for three years at sea and in James Cook, it had one of the foremost seamen and navigators of history. Also on board were three scientists: Charles Green, the assistant to the Astronomer Royal, Joseph (later Sir Joseph) Banks, the President of the Royal Society, and Dr. Solander, an able biologist.

The principal intention of the expedition was to observe the transit of Venus on 1769 June 3 from the Pacific (only the start of the event was visible from London). After the observation, Cook was to make whatever discoveries he could in the South Seas. As it turned out, Cook made the observation of the transit of Mercury in Tahiti, went on to discover New Zealand (making an observation of the transit of Mercury on 1769 Nov 9 near present-day Auckland), and continued on to explore the uncharted east coast of Australia.

What was so important about the transit of Venus? Also, what is a transit? To answer the second question first, a transit is the passage of a smaller object in front of a larger one. Only the planets Mercury and Venus can transit the Sun – Mercury transits about 13 times per century; Venus transits less often, only a couple of times per century. [There were transits of the Sun by Venus in 1874, 1888, none in this century, and there will be transits in 2004 June 8, and in 2012 June 6.]

To answer the initial question, one has to understand the determination of distances in the Solar System. Since the time of Copernicus, astronomers have known the relative distances of the planets from the sun (ie. a scale drawing could be made). However, the absolute distance between any two objects in Earth units (eg the kilometre) was much harder to obtain – only rough estimates could be made. Therefore the astronomical unit (the mean distance of the Earth from the Sun) was only approximately known. If one could determine precisely the distance between the Earth and another planet at closest approach, one could calculate all the other distances.

The transit of Venus was deemed to be a rare opportunity to determine such a distance precisely. Here’s how it was supposed to work: observers at different places on the Earth were to record the instant at which an edge of Venus touched the edge of the Sun (either externally or internally) to within a few seconds. If the distance between observers was around 6000 km or more, time differences of between 5 and 15 minutes (depending on the geometry of the situation) should be detected. By the appropriate calculations, it was hoped, astronomers would be able to determine the distance to Venus and hence, to the Sun.

In the event, unfortunately, James Cook’s chronometers were fast by about four minutes and the results were useless (just try to find a reference to the event in any astronomical text!). By fortunate circumstance, I have obtained a reprint of James Cook’s diary (published 1925) and, with the aid of Guide 2.0 software, I can accurately recreate both transits. In doing so, we can learn about the observations that James Cook and his scientific passengers made. A presentation will be made at the February meeting using the observatory’s computer and whatever else I can get together.

Monthly Meetings

Last Meeting

At the last meeting of the PGAS, Bob presented another excellent constellation of the month. Then, after some trouble with the video player, the first half of John Dobsen’s tape on Telescope Making was played. The cookies were enjoyed by everyone; lets hope that Orla remembers to buy some more for the next meeting.

At the start of the meeting, Orla summarized some of the activities that the club will be involved with this year: Among them were

  • Science Fair Tour/CNC Booth April 8
  • Astronomy Day April 16
  • AAPT/Articulation Tour May 6 & 7
  • Public tours May 1 to June 25
  • Canada Day Participation? July 1 & 2
  • Grand opening (the Q subject) August 17

Next Meeting

The next meeting will be held at CNC in the Physics Laboratory (room 2-223) on February 23rd. The meeting will start at 7:30 PM with a short business reports. Afterwards, Bob will give a more detailed report about the astronomical observations of James Cook, Jon will show some of the Hubble computer images recently obtained from JPL, and finally John Dobson will conclude their presentation on telescope making.



Our new digital setting circles have arrived. They are still being tested, and we may have some results by the next meeting. Hopefully, we will have pointing accuracy to within one field width in the near future.

Observatory Computer

When you go out to the observatory, please take care NOT to use the computer if the temperature is below freezing. Damage to the hard drive may occur. Thank you.

Hubble Images Available

Jon Bowen and Orla Aaquist have retrieved some of the latest Hubble images from NASA and various other places. The images are transferred to CNC via electronic mail from the public information area on the JPL computer. If you would like to see some of these images, or obtain GIF files of these images, give Orla a call at work (562-2131 local 307) or at home (964-9626).

Photographs Wanted

The PGAS needs photographs for our portfolio. If you have any which show the construction of the observatory or of our public activity, please contact Jon Bowen or Orla Aaquist.

Free Space for Announcements

If you have anything to announce or advertise, consider this space of the PeGASus.

DIAMONDS IN THE SKY by Matthew Burke

During the period between July 16, 1994 to July 22, 1994 the major fragments of the comet Shoemaker-Levy 9 will collide on the dark side of Jupiter. Pictures of the comet show specks of light travelling in a row resembling a string of diamonds. Depending on the size of the fragments, the collisions could be the most spectacular event ever witnessed by Astronomers.

The comet Shoemaker-Levy 9 was discovered by Carolyn Shoemaker, Eugene Shoemaker, and David Levy at the mount Palomar Observatory on March 25, 1993. This is the 9th comet the team has discovered. David Levy is credited with 19 comet discoveries which ranks him the second most successful living comet hunter, after Carolyn Shoemaker who has discovered 30 comets. Shoemaker-Levy 9 is a 13.8 magnitude object and was found on photographic plates taken using an 18″ Schmidt camera. The comet is believed to have been in orbit around Jupiter for 20 years, with the aphelion at about 0.33 AU (note one Astronomical Unit = 149,600,000 Km) and an eccentricity over 0.995. On July 7, 1992 it came 94,000 Km to the centre of the planet (within the Roche limit) tearing it apart into the 21 major fragments seen today. The progenitor comet size is believed to be 10 Km in diameter, although sizes of 20 Km – 2 Km are also speculated. The comet fragments are estimated to be between a fraction of kilometre up to 5 Km in diameter. More accurate observations by the Hubble Space Telescope will confirm the actual sizes.

The size of the fragments is very important because the kinetic energy released is equal to 1/2 of the mass times the velocity squared. At time of impact the speeds are estimated to be around 60 Km/sec. An object with a mass (in grams) of 10 to the eighth power will release energy at a peak rate of .02 megatons of TNT per second yielding an apparent visual magnitude of +10. While an object with a mass of 10 to the power of sixteen grams will release energy at a peak rate of 100,000 megatons of TNT per second yielding an apparent visual magnitude of -10 (Science – Vol. 262 – 15 October 1993).

The new impact estimates from Sekenina, Paul Chodas, and Donald K. Yeomans (of Jet Propulsion Laboratories) are located five to nine degrees behind the limb, – 44 degrees Jupiter latitude @ 67 degrees east (towards the sunrise terminator) from the midnight meridian. At this location any explosions reaching 1000 Km above the cloud tops of Jupiter will be directly observable. Although the impact locations are out of sight of earth bound eyes, the spacecraft Galileo @ 1.34 AU from Jupiter, Voyager 1 @ 52 AU, Voyager 2 @ 41 AU, and Ulysses will be in position to directly observe the impacts. Unfortunately software controlling the on- board cameras on both Voyager 1 & 2 has been deleted making imaging the impacts in the visual spectral range unlikely. The only Voyager instruments likely to observe the impacts are the ultraviolet spectrometers and planetary radio astronomy instruments. The Ulysses spacecraft will monitor the event in radio waves. Unfortunately Galileo’s antenna array is damaged, limiting its data transition rate at a pitiful 10 bits per second. Galileo will be the only spacecraft to directly image the impacts.

Comets are thought to be primordial balls of frozen gas, ice, and dust formed in the outer solar system and are often described as “dirty snowballs”. Generally they are composed of gases (ammonium, carbon dioxide, carbon monoxide, methane), water (70-80%), and dust: (20-30% metals & rocky material). Comets travel in highly elliptical orbits around the sun and can have orbital periods of tens of years to tens of thousands of years. Comet tails are formed by vaporized dust and gas swept back by the pressure of solar light and can be millions of kilometres long.

tres long. Jupiter is the largest plant in the solar system, with more mass than all other planets combined. It is composed of mostly hydrogen and helium with a bulk density slightly higher than water. Jupiter orbits at an average distance of 5.2 AU from the sun, and takes 12 earth years to orbit the sun. A day on Jupiter lasts 9 hours 55 minutes. The planet has an equatorial radius of 71,398 Km, and the second most extensive system of moons in the solar system with 16.

Stay tuned for next months issue when I report on detailed predictions of the collision effects, and local times of the collisions.

References Astronomy, Vol 22 Issue 1 Jan 1994 page 19 Astronomy, Vol 21 Issue 12 Dec 1993 page 18 Astronomy, Vol 21 Issue 9 Sep 1993 page 18 Nature, Vol 140 Issue 1898 Nov 6 , 1993 page 18-19 Nature, Vol 356 Issue 6448 Oct 21, 1993 page 731-733 Nature, Vol 363 Issue 6429 Jun 10, 1993 page 492-493 Science, Vol 262 Issue 5133 Oct 22, 1993 page 505 Science, Vol 262 Issue 5132 Oct 15, 1993 page 382-387 Science, Vol 261 Issue 5121 Jul 30, 1993 page 552 Scientific America, Vol 269 Issue 3 Sep 1993 page 26-30 Science News, Vol 143 Issue 26 Jun 26, 1993 page 410 Natural History, Vol 102 Issue 10 Oct 1993 page 40-42 Maclean’s, Vol 106 Issue 31 Aug 2, 1993 Page 42 Sky & Telescope, Vol 86 Issue 1 Jul 1993 page 38-39 World Book Information Finder (c) 1993 World Book Inc (Jupiter, Comet) Comet/Jupiter Collision FAQ Last Update 18-Jan-1994 page 1-7

Picture Perfect Photo release number: STScI-PR94-02 figure caption (modified)

The image below is of the spiral galaxy M100 obtained using the second generation Wide Field and Planetary Camera (WFPC-2), installed in the Hubble Space Telescope during the STS-61 Servicing Mission. The WFPC-2 contains modified optics to correct for the aberration of the Hubble’s primary mirror. The new optics will allow the telescope to tackle many of the most important scientific m100_aprograms for which the it was built, but had to be temporarily shelved with the discovery of the spherical aberration in 1990.

The picture shows a mosaic of the images taken with WFPC-2s four separate cameras. The three Wide Field cameras give HST its “panoramic” view of astronomical objects. The Planetary Camera, has a smaller field of view but provides better spatial resolution. The image shows the fields of view of the four cameras combined into a “chevron” shape, the hallmark of WFPC-2 data. The three wide field detectors in the camera reveal individual stars and filamentary dust lanes in the outer arms of the majestic spiral galaxy. The instrument’s planetary camera image (upper right) resolves complex structure in the core of the galaxy, which is the site of vigorous star formation. The image was taken on December 31, 1993. The field of view is about two and a half arc minutes across.

Though the galaxy lies several tens of millions of light-years away, modified optics incorporated within the WFPC-2 allow Hubble to view M100 with a level of clarity and sensitivity previously possible only for the very few nearby galaxies that compose our Local Group. Just as one does not learn about the diversity of mankind by conversing only with your next door neighbour, astronomers must study many galaxies in a host of different environments if they are to come to understand how our own galaxy, our star, and our earth came to be. By expanding the region of the universe that can be studied in such detail a thousand fold, the WFPC-2 will help the Hubble Space Telescope to fulfil this mission.

One of the greatest gains of the high resolution provided by Hubble is the ability to resolve individual stars in other galaxies. The new camera not only allows astronomers to separate stars which would have been blurred together at the resolution available from the ground, but also allows astronomers to accurately measure the light from very faint stars. The quantitative study of compositions, ages, temperatures, and other properties of stars and gas in other galaxies will provide important clues about how galaxies form and evolve.

In addition, the WFPC-2 will allow the Hubble Space Telescope to be used to attack one of the most fundamental questions in science: the age and scale of the universe. Astronomers have many ‘yardsticks’ for measuring the scale of the universe, but lack a good knowledge of how long these yardsticks really are. M100 is a member of the Virgo Cluster of galaxies. By allowing astronomers to resolve and measure individual stars in the Virgo Cluster (in particular a special type of star called Cepheid variables, which have well known absolute brightness values) HST observations are expected to provide a crucial measurement of this much needed scale. Only the Space Telescope can make these types of observations. Cepheids are too faint and the resolution too poor, as seen from ground-based telescopes, to separate the images in such a crowded region of a distant galaxy.

Astrology Survey

A York Astrology/Astronomy Survey (RASC Victoria SkyNews No. 149) reports that a survey of over 1500 first year university Arts and Science students reveals that approximately 40% “subscribe at least somewhat to the principles of astrology”, and over half are unable to distinguish between astronomy and astrology. The response from the Science students are not markedly different from the Arts students.

The Image Gallery

melnick34The three images below were retrieved from by Jon Bowen. The three panels show images of a very bright Wolf-Rayet star, Melnick 34, located in the Large Magellanic Cloud. In the background are a number of fainter stars that are comparable in brightness to our Sun. The first image is the best available ground-based image of the region. This image was taken under ideal atmospheric conditions (0.6″ resolution). The second image shows the spherical aberration which was present before the repairs were made. In particular, there is a 4″ skirt around the bright star which obscures the view of the sky in its vicinity. The last image was taken with the improved optics. As can be seen, the aberration is gone from the central star. Note that a large number of fainter stars also become visible. This is because their light is now concentrated in the star’s image rather than in a halo. This makes quantitative measurements of their brightness possible.

PeGASus is published monthly by the Prince George Astronomical Society. Contributions to the newsletter are welcome.

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